﻿namespace Devonline.Core;

/// <summary>
/// 雪花算法
/// </summary>
public class SnowFlake
{
    /// <summary>
    /// 唯一时间随机量
    /// 唯一时间，这是一个避免重复的随机量，自行设定不要大于当前时间戳
    /// </summary>
    private const long twepoch = 9527000L;
    /// <summary>
    /// 数据中心码
    /// </summary>
    private static long datacenterId = 0L;
    /// <summary>
    /// 机器码
    /// </summary>
    private static long machineId = 0L;
    /// <summary>
    /// 计数序号, 计数从零开始
    /// </summary>
    private static long sequence = 0L;

    /// <summary>
    /// 机器码字长
    /// </summary>
    private const long machineIdBits = 5L;
    /// <summary>
    /// 数据中心码字长
    /// </summary>
    private const long datacenterIdBits = 5L;
    /// <summary>
    /// 计数器字长，12个字节用来保存计数码
    /// </summary>
    private const long sequenceBits = 12L;

    /// <summary>
    /// 最大机器码
    /// </summary>
    private static readonly long maxMachineId = -1L ^ -1L << (int)machineIdBits;
    /// <summary>
    /// 最大数据中心码
    /// </summary>
    private static readonly long maxDatacenterId = -1L ^ (-1L << (int)datacenterIdBits);

    /// <summary>
    /// 机器码数据左移位数，就是后面计数器占用的位数
    /// </summary>
    private static readonly long machineIdShift = sequenceBits;
    /// <summary>
    /// 数据中心左移位数
    /// </summary>
    private static readonly long datacenterIdShift = sequenceBits + machineIdBits;
    /// <summary>
    /// 时间戳左移动位数就是机器码+计数器总字节数+数据字节数
    /// </summary>
    private static readonly long timestampLeftShift = sequenceBits + machineIdBits + datacenterIdBits;
    /// <summary>
    /// 一毫秒内可以产生计数，如果达到该值则等到下一微妙在进行生成
    /// </summary>
    public static readonly long sequenceMask = -1L ^ -1L << (int)sequenceBits;
    /// <summary>
    /// 最后一次的时间戳
    /// </summary>
    private static long lastTimestamp = -1L;
    /// <summary>
    /// 加锁对象
    /// </summary>
    private static readonly object syncRoot = new();
    /// <summary>
    /// 基准时间, 时间的起始值
    /// </summary>
    private static readonly DateTime startTime = new(2020, 1, 1, 0, 0, 0, DateTimeKind.Utc);
    /// <summary>
    /// 单实例对象
    /// </summary>
    private static SnowFlake? snowflake;

    /// <summary>
    /// 获取默认实例
    /// </summary>
    /// <returns></returns>
    public static SnowFlake Instance()
    {
        if (snowflake == null)
        {
            snowflake = new SnowFlake();
        }

        return snowflake;
    }

    /// <summary>
    /// 默认构造方法
    /// </summary>
    public SnowFlake()
    {
        Snowflakes(0L, -1);
    }
    /// <summary>
    /// 只包含机器码的构造方法
    /// </summary>
    /// <param name="machineId"></param>
    public SnowFlake(long machineId)
    {
        Snowflakes(machineId, -1);
    }
    /// <summary>
    /// 包含机器码和数据中心码的构造方法
    /// </summary>
    /// <param name="machineId"></param>
    /// <param name="datacenterId"></param>
    public SnowFlake(long machineId, long datacenterId)
    {
        Snowflakes(machineId, datacenterId);
    }

    /// <summary>
    /// 初始化机器码和数据中心码
    /// </summary>
    /// <param name="machineId"></param>
    /// <param name="datacenterId"></param>
    private static void Snowflakes(long machineId, long datacenterId)
    {
        if (machineId >= 0)
        {
            if (machineId > maxMachineId)
            {
                throw new Exception("机器码码非法");
            }

            SnowFlake.machineId = machineId;
        }

        if (datacenterId >= 0)
        {
            if (datacenterId > maxDatacenterId)
            {
                throw new Exception("数据中心码非法");
            }

            SnowFlake.datacenterId = datacenterId;
        }
    }

    /// <summary>
    /// 生成当前时间戳
    /// </summary>
    /// <returns>毫秒</returns>
    private static long GetTimestamp()
    {
        return (long)(DateTime.UtcNow - startTime).TotalMilliseconds;
    }

    /// <summary>
    /// 获取下一微秒时间戳
    /// </summary>
    /// <param name="lastTimestamp"></param>
    /// <returns></returns>
    private static long GetNextTimestamp(long lastTimestamp)
    {
        int count = 0;
        long timestamp = GetTimestamp();
        while (timestamp <= lastTimestamp)
        {
            //这里获取新的时间,可能会有错,这算法与comb一样对机器时间的要求很严格
            count++;
            if (count > 10)
            {
                throw new Exception("机器的时间可能不对");
            }

            Task.Delay(1);
            timestamp = GetTimestamp();
        }

        return timestamp;
    }


    /// <summary>
    /// 获取长整形的ID
    /// </summary>
    /// <returns></returns>
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1822:Mark members as static", Justification = "<Pending>")]
    public long GetId()
    {
        lock (syncRoot)
        {
            long timestamp = GetTimestamp();
            if (lastTimestamp == timestamp)
            {
                //同一毫秒中生成ID
                //用&运算计算该微秒内产生的计数是否已经到达上限
                sequence = (sequence + 1) & sequenceMask;
                if (sequence == 0)
                {
                    //一毫秒内产生的ID计数已达上限，等待下一微妙
                    timestamp = GetNextTimestamp(lastTimestamp);
                }
            }
            else
            {
                //不同微秒生成ID
                sequence = 0L;
            }

            if (timestamp < lastTimestamp)
            {
                throw new Exception("时间戳比上一次生成ID时时间戳还小，故异常");
            }

            //把当前时间戳保存为最后生成ID的时间戳
            lastTimestamp = timestamp;
            long Id = ((timestamp - twepoch) << (int)timestampLeftShift)
                | (datacenterId << (int)datacenterIdShift)
                | (machineId << (int)machineIdShift)
                | sequence;

            return Id;
        }
    }
}